Biological characteristics of Conger myriaster during the initial stage of spawning migration in the East China Sea

The biological characteristics of Conger myriaster in the East China Sea were studied in May and June (summer), and in November and December (winter), from 2009 to 2010. C. myriaster was collected mainly at stations in the survey area contained within longitudes 125°E and 128°E and latitudes 28°N and 33°N. Depths and bottom water temperatures for the positive stations ranged from 73 to 186 m and 12.5 to 19.2 °C in summer, and from 65 to 187 m and 13.8 to 21.4 °C in winter. A total of 107 specimens included 64 pre-vitellogenesis (peri-nucleolus to early oil droplet stages) females, 16 mid-vitellogenesis (primary to secondary yolk globule stages) females, 24 pre-meiosis males (spermatogonial proliferation stage), and three sex-unidentified specimens. The gonads of summer-collected mid-vitellogenesis females were more developed than those of winter-collected mid-vitellogenesis females. Additionally, mid-vitellogenesis females were collected mostly where bottom water temperature was lower than 20 °C in both summer and winter. On the other hand, males were captured only in winter and were all pre-meiosis individuals. This suggested that C. myriaster has different behaviors between females and males. Furthermore, sex ratios (female:male) were 46:0 in summer and 34:24 in winter. The presence of mid-vitellogenesis females and the absence of males in summer suggested different spawning migration routes and/or seasons for females and males.